Our previous work has demonstrated that the vestibular system has powerful influences on the sympathetic nervous system, whose major function is the control of circulation. The properties of vestibulo-sympathetic reflexes are appropriate to compensate for posturally-related changes in blood pressure. Furthermore, a large fraction of the brainstem neurons that regulate circulation also receive vestibular signals. Thus, a major function of vestibula influences on sympathetic outflow appears to be maintenance of stable blood pressure during movement. The present application will more thoroughly investigate the physiological role of vestibulo-sympathetic connections. Recordings from sympathetic neurons and sympathetic nerves will determine if vestibular influences are confined to those cells that regulate circulation. Recordings of blood flow during vestibular stimulation will directly test whether the vestibular system has strong cardiovascular effects. Recordings of blood pressure in labyrinthine-deficient animals will determine if the vestibular system plays a significant role in compensating for postural hypotension. The proposed work will also examine the integration of inputs from the neck and labyrinth in producing sympathetic responses during head movement. In addition, the relative contribution of semicircular canals and otolith organs to vestibulo-sympathetic reflexes will be analyzed. Many people have difficulty in maintaining constant blood pressure during movement and changes in posture, and it is possible that deficiency in vestibulo-sympathetic responses contributes to this condition. The present grant will examine this possibility, and provide basic science insights that will be important in providing treatment for this problem.